The present invention relates to an angular motion measurement arrangement and, in particular, it concerns an angular rate sensor arrangement for a seeker head, which is part of a guided missile.
A guided missile includes a seeker head, which includes a sensor that is configured to sense a target. The seeker head is typically gimbaled, whereby an actuator is configured to ensure that the sensor always points in the direction of the target. Measurements of the angular velocity of the seeker head together with the seeker's measurements are then used to guide the missile toward the target.
The guided missile generally includes an inertial measurement unit (IMU) which measures the motion of the missile with respect to an inertial reference frame. Measurements made by the IMU in conjunction with measurements of the relative angle between the seeker head and the missile body could be used to calculate the angular velocity of the seeker head. Even though the IMU can provide measurements which are accurate with low noise and long term stability, the IMU cannot measure the high frequency dynamics of the guided missile and the seeker head needed for the stabilization of the seeker head. Therefore, a separate angular velocity measurement device is needed to measure directly the angular velocity of the seeker head. The angular velocity measurement device is typically based on one or more gyroscopes. The measured angular velocity of the seeker head is then used to guide the missile toward the target.
As discussed above the gyroscope(s) used in the angular velocity measurement device need to have a high bandwidth. Additionally, they need to give accurate results with low noise. Inexpensive gyroscopes may have a high bandwidth, but they lack long term accuracy in two aspects. First, they have a high bias such that, even when the seeker head is not moving they measure a very high angular velocity that can reach 100 degrees per second or more. Second, they have a bad scale factor that can reach an error of more than 20%. Therefore, a gyroscope having a low bias, good scale factor, low noise and high frequency bandwidth is needed in order to measure the angular velocity of the seeker head. Such a gyroscope can be expensive, approximately, $3,000 to $5,000 each.
There is therefore a motivation to provide a system for measuring the angular motion of a seeker head which has a low bias, good scale factor, low noise and high frequency bandwidth at a low price.